The present invention relates to a radiation detecting device for detecting X rays, .gamma. rays, etc., and a method for fabricating the same.
The most typical one of the techniques using radiation, such as X rays, .gamma. rays, etc. uses synchrotron radiation. The synchrotron radiation is electromagnetic waves of very strong directivity emitted when electrons which are emitted from a synchrotron and moving at velocities approximate to the light velocity are deflected from their orbits by a magnetic field. Synchrotron radiation is so superior in wavelength continuation, deflectivity, pulsation, etc. that its applications in a very wide variety of fields, such as crystal structure analyses, surface structure analyses, interface structure analyses, property studies, imaging techniques, industrial applications, biological applications, medical applications, etc. are expected.
Not only synchrotron radiation, but also other radiation, such as common X rays, etc., are widely used in medical cares and property analyses.
The radiation detecting devices used in these fields have detecting units of various structure and detecting characteristics, etc. in accordance with their applications. A factor of the radiation detecting devices which is important commonly in all these fields is good energy resolving powers.
As radiation detecting devices of high energy resolving powers are known are semiconductor radiation detecting devices using semiconductors, such as Si, Ge and others. These energy resolving powers have almost reached to limits defined by statistical fluctuations of electron-hole pair numbers created therein by the applied radiation.
In some of the above-described application fields, radiation detecting devices of higher energy resolving powers than theoretical limit energy resolving powers of semiconductor radiation detecting devices are earnestly expected. To this end, radiation detecting devices whose limit energy resolving powers due to statistical fluctuations are low are demanded. What is noted under such circumstances is radiation detecting devices using superconductors.
When radiation of energy E is incident on a radiation detecting device, a number of electron-hole pairs are created and are collected as detection signals. A relative energy resolving power P caused by fluctuations of a number N of created electron-hole pairs is given by the following formula: EQU P=2.355 (.epsilon..times.F.times.E).sup.1/2
where .epsilon. is energy necessary to create one electron-hole pair by radiation; and F is called Fano factor and defined by &lt;(N-&lt;N&gt;).sup.2 &gt;/&lt;N&gt;, and F is a parameter for a degree of fluctuations of a number of the excited electrons being smaller than a Poisson's distribution, and F=1 when they agree with a Poisson's distribution and F&lt;1 when they do not agree with a Poisson's distribution ("Energy Dispersiton type X-ray Analysis" edit by Yohichi Gohshi and Kimitaka Satoh, Nippopn Bunkoh Gakkai Measuirng Method Series, Gakkai Shuppan Center).
This formula shows that it is an important factor to improve an energy resolving power of a radiation detecting device that a material which is able to create a larger number of electron-hole pairs by incident radiation, i.e., a material which has a small energy .epsilon., is available.
It can be said that electron-hole pairs can be created with a small energy .epsilon. when a material of a radiation detecting device has a narrow energy gap Eg. But, when a semiconductor material is used, an energy .mu. is expressed by EQU .epsilon.=2.8.times.Eg+(0.5.about.1.0) [eV].
Thus it is known that a small energy gap Eg does not always result in a proportionally small energy .epsilon. value (C. A. Klein, J. Appl. Phys., vol. 39, p. 2090, (1968)).
That is, this formula shows that however small an energy gap Eg may be, an energy .epsilon. is never below 0.5.about.1.0 eV, i.e., that a number of created electron-hole pairs is limited. Thus in radiation detecting devices of semiconductor materials, phonons (with a maximum energy of tens meV) emitted when electron-hole pairs are generated can not excite electron-hole pairs.
But in radiation detecting devices of superconductors, because of small energy gaps Eg (about some meV), phonons (with a maximum energy of tens meV) when electron-hole pairs are broken break electron pairs to generate quasiparticles, i.e., increase a number of quasiparticle N, and it is possible to fabricate radiation detecting devices of high energy resolving powers.
A Monte Carlo simulation was conducted by a computer on, in a radiation detecting device using a superconductor material, a process of quasiparticle and phonon excitation of a cascade excitation of direct electron excitation of radiation, emission of phonons by quasiparticles, and creation of quasiparticles by the phonons, and the result was that a number of quasiparticles which is larger by three figures than that in semiconductor radiation detecting devices, and energy .epsilon.=0.969 meV, and a parameter F=0.2 were theoretically given (M. Kurakado and H. Mazaki, Phys. Rev. B22 (1980) 168).
In experiments on Sn/SnOx/Sn junctions which will be explained later, energy .epsilon.=2.4 meV (M. Kurakado, Nucl. Nuc. Instrum and Method, vol 186, p 275 (1982). FIG. 23 shows relationships between energy resolving powers and incident energy with a Fano factor F and energy .epsilon. given by theoretical values for Case 1, and with energy .epsilon. given by an experimental value and F=1. In FIG. 23, a relationship between energy resolving powers and incident energy for a case in which Si, which has the highest energy resolving power among the semiconductor materials, (.epsilon.=3.76 eV, F=0.08 (experimental values)) is shown for comparison.
As shown in FIG. 23, it is possible that the use of a superconductor materials can fabricate radiation detecting devices having energy resolving powers higher by about 10 times than those of radiation detecting devices using semiconductor materials.
The tunneling process of a Josephson junction comprising a superconductor 1/a barrier/a superconductor 2 occurring when radiation is applied thereto will be explained with reference to FIG. 24. FIG. 24 shows a state in which with a voltage applied to the Josephson junction, the superconductor 1 on one side has a higher energy band by eU.
By the incidence of radiation on the superconductor 1 on one side, electron pairs are broken to create quasiparticles. The creation of the quasiparticles is due to direct excitation of the radiation or excitation by phonons. The qausiparticles reach the superconductor 2 through a tunnel of the barrier.
Improvement of the energy resolving power depends on efficiency of the created quasiparticles tunneling the insulator. This tunneling phenomenon is a non-equivalent state, and efficiency of the tunneling is determined firstly by a relationship between a tunneling time of the quasiparticles, and a recombining time of the quasiparticles accompanied by creation of phonons. The tunneling time and recombining time of the quasiparticles greatly vary depending on kinds of the superconductors. Secondly the efficiency is determined by making the thermally excited quasiparticles less influential, and it will be necessary to measure the radiation at lower temperatures (&lt;4.2K).
The tunneling process of the quasiparticle in the Josephson junction exhibits the current-voltage characteristic of FIG. 25. That is, when a voltage Vo is smaller than a value 2.DELTA./e corresponding to a bandgap energy 2.DELTA., only quasiparticles above an energy band, and their holes contribute to the current.
In a constant-voltage type in which a constant-voltage Vo is applied to the Josephson junction, a current variation .DELTA.I is generated before and after incidence of radiation, and the current variation .DELTA.I is taken out as a signal. In a constant-current type in which a constant-current Io is applied to the Josephson junction, a voltage variation .DELTA.V is generated before and after incidence of radiation, and the voltage variation .DELTA.V is taken out as a signal. These current variation .DELTA.I and voltage variation .DELTA.V are proportional to incident energy of the radiation.
Radiation detecting devices using such superconductor materials have potential performances surpassing radiation detecting devices using semiconductor materials and are found very prospective.
Conventional radiation detecting devices using superconductors will be explained.
An experiment for applying radiation to a Josephson junction was conducted about 20 years ago (C. H. Wood and B. L. White, Appl. Phys. Lett., vol. 15 (1969), p. 237). The junction comprises Sn/SnOx/Sn. In this experiment signals generated by application of .alpha. radiation at most could be identified.
This experiment was followed by a string of experiments using the same Sn/SnOx/Sn junction (M. Kurakado, S. Tachi, R. Katano and H. Mazaki, Bull. Inst. Chem. Res., 59 (Kyoto Univ. 1981) etc.). These experiments found that signals generated by radiation application are not attributed simply to temperature rises, but creation of surplus quasiparticles by the radiation application is the substance of generation of signals by the Josephson junction.
These experiments have been followed by studies and developments of superconductor radiation detectors all over the world in connection with problems of astrophysical problems and elementary particle physical problems, such as determination of masses of neutrinos, measurement of energy distributions of solar neutrinos, and detection of dark substances and magnetic monopoles.
In 1986 Kraus et al. (H. Kraus et al., Europhys. Lett., 1 (1986) 161) and Twerenbold (D. Twerenbold, Europhys, Lett., 1 (1986) 209) independently succeeded in detection of MnK.alpha. and MnK.beta. from .sup.55 Fe by X rays. The Josephson junction materials were Sn/SnOx/Sn, and the measuring temperature was 0.3K for the preclusion of thermal influences. According to Twerenbold et al.'s experiment, an energy resolving power was a 90 eV half-width for 5.9 KeV X-rays application, and the energy resolving power was a 90 eV half-width excluding the influence of the energy diffusion to the wiring, and was a 41 eV half-width excluding a width due to noises.
Later, by reducing the wire width down to 4 .mu.m were obtained an energy resolving power of a 48 eV half-width for the application of 5.9 KeV radiation, and an energy resolving power of a 37 eV half-width excluding a width due to noises (W. Rothmund and A. Zohnder, Superconductive Particle Detectors, edited by A. Barone (Work Scientific, Singapore)).
It was proved that radiation detecting devices using such superconductor materials are superior in performance to radiation detecting devices using semiconductor materials. But the energy resolving powers of these radiation detecting devices are larger by about one figure than 2.5 eV, the theoretical value estimated based on the energy gap of Sn. More improvement is required in their device structures. Furthermore, Fatally Sn is vulnerable to the heat cycle of the room temperature and the helium temperature, and tends to be degraded even during storage at the room temperature.
In view of this, development is under way on Josephson junctions using materials invulnerable to the heat cycle. Barone et al. showed that the use of Pb/NbOx/Nb junction with Nb, which is strong to the heat cycle, used as the lower electrode, and a reduced wire width can provide sharp pulse height spectra (A. Barone et al., Nucl. Instrum. and Methods, A234 (1985) 61, and others).
Applications of Nb/AlOx--Al/Nb junction, which has been successful in digital applications, in radiation detecting devices and other applications are tried (M. Kurakado and A. Matsumura, J.J. Appl. Phys., 28 (1989), L459; Japanese Laying-Open Hei 01-122179 (1989); and others). But their characteristics are not good in comparison with Sn/SnOx/Sn junction. To improve the characteristics, Kurakado et al. monocrystallized the lower electrode of Nb, whereby an energy resolving power of a 160 eV half-width was obtained for the application of 5.9 keV X rays (Kurakado et al., Technical Report of the Institute of Electronics and Communication Engineers, SCB90-19, pp 7). But the characteristics are still not good in comparison with Sn/SnOx/Sn junction.